Chuckles is right concerning battery monitors. Voltage alone only gives you a very rough idea of battery state. To be effective it has to measure voltage on a "rested" battery, which doesn't happen on a boat. However, if it's what you have to go on... A fully-charged battery should show 12.7 or 12.8 volts. The voltage will drop as you use the battery. At 12.2 volts the battery has only 50% charge and should be charged again. I also use voltage on my boat to monitor batteries, but since I make more power than I use, I don't really worry too much about input-output. My two 75w panels top up the batteries during the day (12.8v) and I might see a voltage before sunrise of 12.6 or 12.5 volts. Not very sophisticated, but it works. You might want to add solar panels and/or wind power and then see how much power you can make. Then tailor your consumption to your power generation. The coffee machine may have to go ...